This invention relates to a two cycle crankcase precompression type of loop scavenging engine having means to compensate for unequal fuel mixture pressures at the scavenging passage entrances caused by the rotation of the crankweb.
Two cycle crankcase precompression engines of the loop scavenging type are commonly used in marine outboard motors, wherein it is essential that the overall engine construction be as compact as possible. Such an engine typically has two or more vertically aligned cylinders and separately partitioned crankchambers, with the fuel mixture being precompressed in each crankchamber during the power stroke of the piston and forceably vented into its associated cylinder scavenging passages whose outlets are uncovered by the piston near the bottom of its power stroke. The scavenging passages are usually disposed on opposite sides of the cylinder at an upwardly converging angle to effect a swirling flow within the cylinder to more effectively scavenge out the exhaust gases and uniformly introduce the new fuel mixture.
To shorten the length of the cylinder block in such an engine in order to achieve a more compact construction, it is known to symmetrically dispose the scavenging passages on the opposite sides of a plane including the crankshaft and cylinder axes, and to orient the exhaust port or ports in a downwardly direction pursuant to an underwater exhaust system. See, for example, U.S. Pat. No. 2,627,255 to Kiekhaefer. In such a construction, however, the rotation of the crankweb in each crankchamber causes a higher pressure at the entrance of one scavenging passage than at the other, which results in inefficient scavenging, non-uniform fuel mixture distribution and degraded combustion and power characteristics. In the full load operation of an outboard motor this problem is particularly serious, since in addition to excessive fuel consumption the supply of lubricating oil is diminished to the sliding surfaces on the weaker scavenging flow side of the cylinder.
Of course, such unequal scavenging passage pressures can be avoided by "rotating" each cylinder 90.degree. so that the passages are symmetrical about a plane perpendicular to the crankshaft, but then the scavenging passages of adjacent cylinders interfere with each other and the length of the cylinder block must be increased to avoid such interference. It is also known to only partially "rotate" the cylinders as disclosed in U.S. Pat. No. 4,075,985. While this avoids scavenging passage interference, some unequal pressure distribution still exists and optimum scavenging efficiency cannot be achieved.